Large enclosures are used as kilns for removing moisture from lumber products by circulation of heated air. For example, green lumber is stacked for drying by placing stickers between each layer of lumber to permit air flow therethrough and the stacks are placed in heated building structures, i.e., kilns, with controlled ventilation and circulation to pass sufficient air through the stacks and carry away the moisture of the lumber.
Most lumber dying kilns rely on internal circulating fans to exhaust the air and replace it with fresh air. For example, by placement of vents on each side of the circulating fans and controllably opening and closing these vents, it is possible to exhaust air from a vent on one side of the circulating fan and draw air into the kiln from a vent on the other side of the circulating fan. When the circulating fans reverse direction, the exhaust vent becomes the intake vent and the intake vent becomes the exhaust vent. Kilns have, therefore, taken into account circulating fan direction and used the circulating fan as a motive force for removing moisture laden air from the kiln and for introducing fresh or make-up air into the kiln. Once the lumber is suitably dried, the stacks are removed from the kiln and further processed or restacked as necessary.
Air is the transport media for picking up moisture at the surface of the lumber product to be dried and moving that moisture to another location for disposal, i.e., exterior of the kiln. It may be appreciated, therefore, that, in order to suitably remove the moisture content of such lumber, it is necessary to monitor the humidity and temperature of air within the kiln. Thus, the manner in which the kiln responds to detected heat and humidity within the kiln plays an important role in the process of kiln drying of lumber.
Such prior kiln systems using internal circulating fans as the motive force for removing moisture laden air are energy inefficient. More particularly, the moisture laden air taken from the kiln is taken just after such air has been heated by the heating element of the kiln. Accordingly, the energy applied to the heating of this air is immediately lost as part of the venting function of the kiln. Also, in such prior systems, tests have shown that as little one-eighth inch change in the vent opening can result in a change of as much as five degrees Fahrenheit in wet bulb humidity measurement. Thus, such prior vent systems cannot provide precise control over internal kiln humidity.
Conventional kiln sensor arrangements and conditions detected thereby include temperature detection by dry bulb and wet bulb sensors whereby a measure of humidity may be calculated. Other kiln condition detection methods include "cellulose" wafers designed to represent the equilibrium moisture content of wood. All kilns, except dehumidification type kilns, vent the moisture laden air to hold a wet bulb condition, i.e., humidity of air within the kiln, down to some desired level. There are computer controlled lumber drying kilns with software configurations providing a variety of control functions.
It is desirable that a kiln system be energy efficient and precise with respect to its control of humidity within the kiln for optimum removal of cooler moisture laden air.